Federal agents and county police are on the hunt for three to four suspects who opened fire on Border Patrol agents in Naco, Ariz., early Tuesday morning. Nicolas Ivie, 30, was fatally shot as he and two other agents were on horseback responding to a tripped motion sensor in a remote mountainous area about five miles from the Mexican border. One of the other agents was shot in the ankle and buttocks and airlifted from the scene to a Tucson hospital for treatment; the third agent was not injured.

“It is obviously an ambush in my mind,” said Cmdr. Mark Denney of the Cochise County Sheriff’s Department in Arizona, which is assisting the FBI in the case. “Someone could have been on high ground and had that over them.”

“There is nothing but cattle, cactus, rocks, and steep canyons,” said Bisbee, Ariz., City Council member Ransom Burke. “That is all there is. [The agents] are well trained, but no one can see between rock and brush. This area is completely geographically ready for an ambush. It has perennially been a drug route for a long time.”

Ivie, who had been with the Border Patrol since January 2008, is the 26th Border Patrol agent to die in the line of duty since 2002 and the second to be murdered in Arizona since 2010. In December 2010, Brian Terry, 40, was killed in a shootout with Mexican hoodlums near the border in Rio Rico, Ariz., about 100 miles away. The guns used in that shootout were later traced to the Fast and Furious gun-smuggling investigation.

“This is a tragic situation,” said Bisbee, Ariz., City Councilman Rob Page. The historic mining town of Bisbee is where Stephen King’s Desperation was primarily filmed. It is located about three miles from Naco, a small incorporated town that boasts a population of around 800 and features a golf course, an elementary school, a small store, and a saloon. “It comes as a shock to me. A lot of my neighbors are Border Patrol agents. It is certainly not something that happens every day.”

“This is just another reminder of our failed border policy, which is endangering all border residents,” said Cochise County Supervisor Richard Searle.

“The news today that a U.S. Border Patrol Agent was killed and one of his partners hurt in a border-enforcement action near Naco, Arizona, is another tragic reminder of the extreme conditions on this portion of the U.S.-Mexico border,” said Arizona Attorney General Tom Horne in a statement.

Denney said the sensors were set off near mile marker 352 on Highway 80, outside of Naco at about 1:33 a.m. Tuesday. “It is an area we have been concentrating on,” said Denney. “We have a lot of areas along the border that seem to be active, and that is one of them. It is very remote and the chances of being encountered are slim because there is no housing or any way you can draw attention to yourself. It is a good area to come through. It would be highly unlikely to be detected.”

The working theory is that the Border Patrol agents most likely came across drug smugglers or human traffickers. “From what we are seeing, if it is human cargo or drugs, it is becoming more volatile,” Denney said. “It is a high-stakes game and there is a lot at stake for those involved. It is one of money and sometimes threats to family members.”

Councilman Burke, a former police officer in Indio, Calif., said that drug loads are usually protected by Mexican drug-cartel shooters. “Their entire purpose is to protect the load,” he said.

Denney said federal law-enforcement officials are still trying to piece together exactly what happened during the early-morning shooting. “Whether [the shooters] were startled or what happened in the encounter, I don’t know,” he said. “I don’t know if the agents fired or not. In this area there are no streetlights. The only ambient light would be only where the moon is casting.”

Denney said Border Patrol agents and the FBI were still scouring the rugged area late Tuesday afternoon looking for evidence. He said his agency is on full alert just in case the shooters are hiding out somewhere in Arizona. Although shootings such as this are rare, Denney says the county is a hotbed of illegal activity. “Our county is a pass-through for those kinds of activities. Our big concern is public safety for our citizens.”

Hampering the investigation is precious little information about the shooters or a description. “I don’t know whether the agents had the opportunity to get a physical description of anyone,” Denney said. “You probably can’t see someone in that type of environment, especially if you are taking cover from being fired upon.”

Sensors going off along the Arizona border are a daily occurrence, said George McCubbin, president of the National Border Patrol Council, a union representing about 17,000 Border Patrol agents. “There is no way to handle the volume of traffic coming in.”

McCubbin said that at this time of year smugglers generally operate at night in Arizona when it is not so hot. “It is still very warm out here, so typically illegal activity occurs at night before the sun comes out. They lay out during the daytime.”

Whether the shooters will be caught is anyone’s guess, says Burke. “If they happen to be able to spot them on this side there is a 50-50 chance to catch somebody,” he said. If they returned to Mexico, “there is zero to no chance they will get caught,” he said.